minor install problems with gdbm-1.8.0

[greep]

Two minor install problems with gdbm-1.8.0:

1. The generated Makefile had "prefix = /usr/local" even
though I specified a different directory using the --prefix 
flag to configure.

2. "make install" tries to set the owner and group of the
installed files.  This fails when not installing as root.

Also, the first argument to gdbm_open() should be declared
const char * instead of char *.